When a surfactant is incorporated in a composition, the surfactant gathers to the interface and reduces the surface tension, and spreadability and permeability are thereby improved.
Fluorine surfactants have, been known to have a surface tension-reducing ability which is superior to that of the corresponding hydrocarbon surfactant or silicone surfactant. Accordingly, fluorine surfactants have been used in a wide variety of fields (see, for example, Patent Literatures 1 to 3).
Examples of the widely used fluorine surfactant include perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and fluorine surfactants based on such surfactants. PFOS has a structure wherein a perfluoroalkyl group is directly bonded to sulfonyl group, and PFOA has a structure wherein a perfluoroalkyl group is directly bonded to carboxy group. In view of the superior performance, perfluoroalkyl groups having a chain length of 8 or more are often selected.
However, due to the extremely hard decomposition of the PFOS and PFOA and high bioaccumulativity of the PFOS and PFOA, global environmental pollution is feared for these substances. In view of the PFOS and PFOA problems, compounds having a perfluoroalkyl group with a chain length of 8 or more have become problematic, and purchase and use of the compound having a perfluoroalkyl group with a chain length of 8 or more are becoming difficult due to the regulatory compliance in various countries and voluntary restraints of companies.
One method for solving the PFOS and PFOA problems is use of a compound having a perfluoroalkyl group with a chain length of less than 8 for the fluorine surfactant. However, the perfluoroalkyl group having a chain length of 8 or more had been preferentially chosen in view of the excellence in the surface tension-reducing ability and leveling property, and the compound having the perfluoroalkyl group with shorter chain length had the problem of significantly reduced surface tension-reducing ability and leveling property.